The present invention relates to cementing operations and, more particularly, to cement compositions comprising particulate carboxylated elastomers and associated methods.
Cement compositions are commonly utilized above ground (e.g., in the construction industry) and in subterranean operations, particularly subterranean well completion and remedial operations. For example, cement compositions are used in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing, cement compositions are pumped into the annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the walls of the well bore. Cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks and holes in pipe strings, and the like.
Once set, the cement sheath may be subjected to a variety of cyclic, shear, tensile, impact, flexural, and/or compressive stresses that may lead to failure of the cement sheath, resulting, for example, in fractures, cracks, and/or debonding of the cement sheath from the pipe string and/or the formation. This may lead to undesirable consequences such as lost production, environmental pollution, hazardous rig operations resulting from unexpected fluid flow from the formation caused by the loss of zonal isolation, and/or hazardous production operations. Cement failures may be particularly problematic in high temperature wells, where fluids injected into the wells or produced from the wells by way of the well bore may cause the temperature of any fluids trapped within the annulus to increase. Furthermore, high fluid pressures and/or temperatures inside the pipe string may cause additional problems during testing, perforation, fluid injection, and/or fluid production. If the pressure and/or temperature inside the pipe string increases, the pipe string may expand and stress the surrounding cement sheath. This may cause the cement sheath to crack, or the bond between the outside surface of the pipe string and the cement sheath to fail, thereby breaking the hydraulic seal between the two. As used herein, the term “bond” encompasses adhesion between surfaces, for example between the cement sheath and the pipe string, on a macroscopic scale and/or attractive forces among portions of molecules on a molecular level, for example, among cement particles and elastomers, and may be ionic, covalent, or the weaker Van der Waals, dipole-dipole types, or any combination of such attractive forces. Furthermore, high temperature differentials created during production or injection of high temperature fluids through the well bore may cause fluids trapped in the cement sheath to thermally expand, causing high pressures within the sheath itself. Additionally, sudden changes in well bore temperatures and/or pressures due to change of fluid densities and temperatures possibly encountered during well bore operations (e.g., construction, remedial operations, fluid injection) subject the cement sheath to cyclic pressure and temperatures, and, if not designed properly, the cement sheath may fail due to its natural brittle properties. Furthermore, failure of the cement sheath also may be caused by forces exerted by shifts in subterranean formations surrounding the well bore, cement erosion, and repeated impacts from the drill bit and the drill pipe.
To counteract these problems, various additives may be included in the cement composition to enable the cement composition to withstand cyclic changes in imposed stresses. For example, hydrocarbon-based elastomers (for example, styrene-butadiene random and block copolymers, acrylonitrile-butadiene, and acrylonitrile-styrene-butadiene elastomers) have been included in cement compositions to modify the mechanical properties of the cement composition. Generally such materials are used in the particulate form. As used herein, the term “particulate” refers to materials in solid state having a well-defined physical shape as well as those with irregular geometries, including any particulates elastomers having the physical shape of platelets, shavings, fibers, flakes, ribbons, rods, strips, spheroids, hollow beads, toroids, pellets, tablets, or any other physical shape. The particulate elastomers may function to control shrinkage cracking in the early stages of the cement setting process, and also may provide resiliency, ductility, and toughness to the set cement composition so that it resists cracking or fracturing. Further, if fracturing or cracking does occur, the particulate elastomers may function to hold the set cement composition together, thereby resisting fall back of the cement sheath. Particulate elastomers also may dissipate stresses more effectively than the cement matrix, thus potentially shielding the cement composition from failing by catastrophic development of fractures and cracks.
The use of particulate elastomers in cement compositions, however, may be problematic. Commonly used particulate elastomers generally contain monomers (such as styrene, butadiene, ethylene, or propylene) that are highly hydrophobic and non-polar. As a result, conventional particulate elastomers are generally non-polar and hydrophobic, while the cement matrix is generally polar and hydrophilic. Due to the hydrophobic nature of conventional particulate elastomers, they generally do not adhere or bond to the cement matrix. Accordingly, the resultant set cement may have a polar and hydrophilic cement matrix with the unbonded and hydrophobic particulate elastomers dispersed therein. The presence of these unbonded particulate elastomers in the cement matrix generally does not allow for the effective transfer of stress from the cement matrix to the particulate elastomers dispersed therein. Additionally, the adhesion of the cement composition to casing and/or formation surfaces may also be compromised due to the poor adhesion of hydrophobic materials to metal and/or formation surfaces resulting in debonding from such surfaces and creating channels for the undesired flow of fluids. Furthermore, addition of such hydrophobic elastomers, which typically have densities either close to or less than that of water, to cement slurries causes them to either float in the slurry or otherwise separate from the cement solids. Addition of the hydrophobic elastomers to the mix water prior to addition of cement may cause the elastomer to float in the mix water so that uniform elastomer distribution into the cement slurry becomes problematic.
Aqueous latex emulsions of elastomeric polymeric materials typically contain small amounts of carboxylic acid containing monomers. For example, styrene butadiene aqueous latex emulsions typically contain small amounts of carboxylic acid containing monomers during polymerization of the styrene and butadiene to provide stability to the aqueous emulsion. However, such latex emulsions may problematic, in that they tend to gel cement compositions and may require large amounts of surfactants to stabilize the cement latex mixtures against premature gelling of cement slurries. Additional problems with the use of latex emulsions in cement compositions include their general lack of stability to the presence of salts and tendency to gel cement compositions at elevated temperatures. Furthermore, aqueous latex emulsions are designed to be film-forming polymer compositions when the water is removed, for example, when the water in cement slurry is consumed in hydration reactions of cement. Such film-forming polymer compositions are not expected to be effective as stress absorbers in cement compositions relative to particulate elastomers which retain their particulate nature even under well bore conditions. Even when a particulate elastomer softens or melts under well bore conditions, it generally remains as a localized softened elastomer or as liquid droplets in the cement matrix instead of forming a film over hydrated cement particles, and thus generally may serve as stress relief sites.